FIG. 1 is a block diagram of a conventional pulsed EMI metal detector and method of operation. A current loop transmitter 10 is placed in the vicinity of the buried metal target 12, and a steady current flows in the transmitter 10 for a sufficiently long time to allow turn-on transients in the soil (soil eddy currents) to dissipate. The transmitter loop current is then turned off. The transmitter current is typically a pulsed waveform. For example, a square-wave, triangle or saw-tooth pulsed waveform, or a combination of different positive and negative current ramps.
According to Faraday's Law, the collapsing magnetic field induces an electromotive force (EMF) in nearby conductors, such as the metal target 12. This EMF causes eddy currents to flow in the conductor. Because there is no energy to sustain the eddy currents, they begin to decrease with a characteristic decay time that depends on the size, shape, and electrical and magnetic properties of the conductor. The decay currents generate a secondary magnetic field that is detected by a magnetic field receiver 14 located above the ground and coupled to the transmitter 10 via a data acquisition and control system 16.
The measurement of these metal object magnetic field decay responses is used to not only detect the metal object but to also classify the metal object. Most metal detected in the environment is not useful. For example, this metallic trash or clutter includes rocks with high ferrous content, and metal from nails and cans. The detection and classification of metal is most important for treasure hunting (coins and jewelry), landmines and unexploded ordnance.
Pulse induction metal detector (PIMD) sensors used for metal detection and classification come in two basic types as shown in FIGS. 2a and 2b. The first type of PIMD uses a single transmit and receiver coil 22 with multiple loops of wire forming the coil (FIG. 2a). A current pulse is sent through the multiple turn coil 22 and the received metal detection signal is sensed by the same coil 22. The small voltage generated by the metal target is typically amplified by a high gain electronic amplifier 25 (typical gain factor of 100 to 1000). A protection circuit is provided to protect the sensitive amplifier from the high kick-back voltage pulse generated by switching the inductive coil off abruptly (V=L di/dt, where L is the inductance of the transmitter coil and di/dt is the slope of the current decay in the coil). The second type of PIMD uses a separate transmitter coil 23 and receiver coil 24, again, with multiple loops of wire forming the coils (FIG. 2b). This configuration provides isolation between the transmitter circuit and the receiver circuit and allows for more flexibility in the receiver coil 24 (e.g., different number of turns, size or differential coil configuration) and amplifier circuit design (e.g., single ended operation of electronics). The high gain amplifier 25 also sees the high kick-back voltage pulse generated by switching the transmitter coil 23 off abruptly and protection circuitry is needed to protect it from damage. After amplification, both types of PIMDs measure the time decay response of the metal object for classification purposes using signal processing techniques known in the art.
Five basic problems exist with prior art PIMDs used for metal detection and classification. First, the high kick-back voltage of the transmitter coil 23 temporally “blinds” the receiver coil 24 from amplifying metal target signals near the turn-off time of the transmitter coil 23. The transmitter coil 23 is an impulse excitation to the receiver coil 24, and as such, the receiver coil 24 will have a decay voltage proportional to the inductance of the receive coil 24. Receiver coils typically have many turns for increased sensitivity and therefore, have relatively large inductances. For metal detectors designed to find low-metal objects such as landmines, these large decay voltages can persist for many microseconds and mask the signal from very small metal targets.
Second, the protection circuitry typically has a delay time that also temporarily “blinds” the receiver coil 24 from amplifying metal target signals near the turn-off time of the transmitter coil 23. Some protection circuitry uses switches to disconnect the receiver coil 24 from the amplifier 25 during the period that the kick-back voltage would cause amplifier saturation or damage. Low noise, high gain, low bandwidth amplifiers take time to come out of saturation which makes them “blind” to metal target signals. Other protection circuitry uses diodes to limit the voltage to the amplifier 25.
Third, a receiver coil 24 will have a voltage decay time proportional to the inductance of the coil 24 that will persist even after the coil comes out of saturation from the transmitter pulse. This residue voltage in the receiver coil 24 limits the amount of amplification that can be used in the receiver amplifier 25 before the amplifier 25 reaches saturation. Large amplifier gain is need to detect small metal objects.
Fourth, the receiver coil 24 residue time decay tend to mask the time decay response of the metal object.
Lastly, the time decay response from the soil can mask the time decay response of small metal objects. This is particularity true of mineralized soil, soil that has electrical and magnetic properties that have a response to electromagnetic induction excitation.
Most PIMD measure a short-term time average voltage from the receiver amplifier and sometimes subtract a long-term time average of the receiver amplifier to balance the ground response and residue receiver voltage. The long-term time average of the balancing amplifier has a time constant that is on the order of a one or two seconds. These type of PIMD do not have the capability to perform target classification based on measuring the time decay response of the metal object. The process of short- and long-term time averaging removes the time decay information in the metal object's response signal.
For PIMDs that measure the time decay signature of the metal object, current methods of canceling unwanted receiver coil response from the transmitter coil transients and mineralized soil include: (1) ignoring the unwanted signals by waiting a sufficiently long time (many micro-seconds) after the transmitter has been turned off and until the unwanted signals have decayed to an acceptable level; and (2) implement a balanced receiver coil arrangement such as a short base-line gradiometer with a single transmitter and two receiver coils.
Neither, however, improve on the detection and classification of metal targets near the transmitter turn-off transient and in the presence of mineralized soil.